class ResultCode {  //正常返回是1  static const SUCCESS = 1;  //异常返回是0  static const ERROR = 0;  /// When opening  url timeout, it occurs.  static const CONNECT_TIMEOUT = -1;  ///It occurs when receiving timeout.  static const RECEIVE_TIMEOUT = -2;  /// When the server response, but with a incorrect status, such as 404, 503...  static const RESPONSE = -3;  /// When the request is cancelled, dio will throw a error with this type.  static const CANCEL = -4;  /// read the DioError.error if it is not null.  static const DEFAULT = -5;}